The field of this invention is automated processing systems used for processing semiconductor wafers, hard disk media, substrates, and similar flat media requiring low levels of contamination. The invention also relates to heaters for solvents and other flammable fluids.
Computers, televisions, telephones and other electronic products contain large numbers of electronic semiconductor devices. To produce electronic products, hundreds or thousands of semiconductor devices are manufactured in a very small space, using lithography techniques on semiconductor substrates, such as on silicon wafers. A large number of individual processing steps may be required to manufacture the semiconductor devices. Various machines and methods have been developed for these applications. For example, U.S. Pat. No. 6,279,724, incorporated herein by reference, describes a system having processing chambers for processing and cleaning flat media (referred to below as xe2x80x9cwafersxe2x80x9d).
In certain processing steps, it is advantageous, or necessary, to apply solvents to the wafers. To speed up and to better control the wafer processing, it is desirable to heat the solvent, and to closely control the temperature of the solvent which is applied to, e.g., sprayed onto, the wafers.
Heating solvents in a safe and reliable way presents unique challenges, because many solvents are combustible. Conventional heating techniques used for other types of fluids are generally unacceptable for heating solvents, due to the risk of igniting the solvent by a malfunctioning electrical heater or heater controller; or because they are unsuitable for the semiconductor manufacturing environment, which must be extremely clean and free of particles; or because they cannot meet the duty cycle requirements needed in semiconductor manufacturing. Quartz heater elements, which have been used to heat various liquids used in semiconductor manufacturing, are unacceptable for heating solvents, because of the risk that the brittle quartz will crack or break, exposing the combustible solvent to extreme temperatures and electrical contacts. On the other hand, tougher materials, such as stainless steel or Teflon, which can reduce or eliminate the risk of breakage of a heating element, and which are also compatible for use with solvents, are unfortunately poor conductors of heat. Accordingly, efficiently heating solvents has remained as a significant engineering challenge.
In the past, blanket heaters have been provided to heat a solvent in a storage tank to a specific set point temperature prior to delivery of the solvent to a wafer processing chamber. The tank blanket heaters are controlled to maintain that set point during processing. While this technique overcomes the difficulties presented by the volatile characteristics of solvents, it has certain disadvantages. Initially, as the entire tank contents must be heated, the desired temperature changes occur slowly. In addition, the tank heaters are controlled based on the solvent temperature in the tank, resulting in significant delays in correcting the solvent temperature back, to the set point, during processing. The heat up time is also long, due to the conduction heating through the stainless steel tank walls, and due to the large mass of stationary solvent. As a result, the temperature of the solvent at the chamber cannot be closely controlled, resulting in poor processing uniformity, low strip/removal rates, and longer process times. Typical temperature drops are 4-8xc2x0 C. from a set point of 75xc2x0 C. In addition, the through put of the system, e.g., in the number of wafers processed per hour, is limited due to the time required for heating the solvent.
Accordingly, there is a need for an improved solvent heater, especially for use in processing semiconductor wafers.
In a first aspect of the invention, in a semiconductor processing machine, a heater for heating solvents includes a solvent tube, and a cooling tube, extending through a casting or other solid form. One or more heating elements extend into the casting. Heat from the heating element is conducted through the casting or solid form, to heat solvent flowing through the solvent tube. As the solvent is isolated from the heating element via the solvent tube and solid material of the casting or solid form, the potential for igniting the solvent during heating is reduced or eliminated.
In a second aspect of the invention, the solvent tube and cooling tube are shaped into coils, with the cooling coil surrounded by the solvent coil. The cooling coil can rapidly cool the casting, if necessary, and can also help to control temperature.
In a third aspect of the invention, the heating element has spaced apart legs which straddle the cooling coil.
In a fourth aspect of the invention, the solvent tube, cooling tube, and heating element are cast in place.
In a fifth aspect of the invention, the casting is enclosed within a container. The walls of the container are insulated from the casting. A purge gas is provided in the space between the insulation and the container walls, to provide an inert atmosphere around the casting.
In a sixth aspect of the invention, cool water is circulated through the cooling tube to rapidly cool the solvent to a temperature low enough to allow the used solvent to be drained from the semiconductor processing machine into the waste lines of a semiconductor manufacturing facility.
Other features and advantages will appear hereinafter.